Factors that determine the quality of liquid coffee and liquid coffee concentrates may be its acidity and/or level of sulfur containing compounds. The effect acids, acidity, and sulfur levels have on the organoleptic characteristics of a cup of coffee can, in many cases, be determining factors in the consumer appeal of a coffee beverage. For example, too much acidity in a coffee can result in an undesirable sourness to the beverage, while too little acidity in the coffee may cause a flat flavor profile. Maintaining a stable acidity and flavor profile of a coffee beverage can, therefore, be important in producing and maintaining a coffee beverage that is desired by consumers.
The task of maintaining the optimal acidity of a coffee beverage is not made any easier because it is believed that over 25 different acids exist naturally in roasted coffee and that many different factors can affect the final acidity of coffee. For instance, coffee may include chlorogenic, malic, citric, acetic, formic, glycolic, lactic and pyroglutamic acids as well as others. In addition, different bean varieties may also affect beverage acidity. For example, the pH of a coffee brewed from Arabica varieties generally has a pH between about 4.85 and about 5.15. Coffee brewed with Robusta beans, on the other hand, generally has a higher pH in the range of about 5.25 to about 5.40. Other processing factors can also affect the degree of acidity, such as, the degree of roast, the roast profile, the nature of the processing and the age of the green beans and the like.
Due to the length of commercial supply chains and the desired shelf life of liquid coffee beverages, a shortcoming exists with liquid coffee beverages. Liquid coffee beverages and extracts tend to be an unstable system, and both shelf- and refrigerator-stored liquid coffee products tend to naturally develop an increased acidity and varying flavor profile over a period of time generally due to naturally occurring acid-generating reactions in the beverage. Over time, these acid-generating reactions can slowly change the beverage's pH throughout a product's shelf file so that the beverage's acidity may also slowly change from the day it is produced to later in its shelf life. A rise in acidity or sourness of the beverage (lowering of pH) may translate into an inconsistent product and a loss of quality.
One attempted solution to prevent the sourness development in liquid coffee beverages over time is the addition of sodium bicarbonate to elevate the initial pH of the product. However, the pH of the sodium bicarbonate-treated liquid coffee product still falls over time as the naturally occurring acid generating reactions occur, which still results in a varying acidity profile throughout the product's shelf life. That is, the product immediately after manufacture may still have a different acidity profile from the product later in the shelf life. The sodium bicarbonate-treated coffee simply starts out with a higher pH that is reduced naturally over time as the reactions occur. However, such treated coffees would still tend to exhibit different acidity characteristics depending on when they are consumed in the product's shelf life. Moreover, the addition of the sodium bicarbonate may have additional potential repercussions on product flavor due to the inclusion of this additional ingredient to the beverage.
U.S. Pat. No. 6,054,162 describes another attempted solution to address the problem of liquid coffee sourness development. The '162 patent describes a method that inhibits the development of acidity by treating a coffee extract with an excessively large amount of alkali relative to the coffee solids to drive a reaction that converts any acid precursors in the beverage to their respective salts to inhibit the generation of acid. However, such method undesirably increases the pH of the coffee to unacceptably high levels with the excessive amounts of alkali (which is needed to drive the desired reactions) and, therefore, also requires neutralization of the treated coffee with an acid to neutralize the excess alkali into respective salts in order to adjust the final pH to the desired value. The excessive amounts of alkali over treat the coffee beverage, which can degrade the flavor and other desired organoleptic characteristics. Moreover, the excessive amounts of alkali require the additional neutralization steps, which add even more pH adjusting steps to the manufacturing process. The method described in the '162 patent requires between 0.1 mol/liter to 0.5 moles/liter of alkali to inhibit the acid generating reactions. On a solids basis, this is between 2 to 10 grams of alkali in an 8 percent coffee solids extract. Put another way, the method of the '162 patent requires 0.25 to 1.25 grams of alkali per percent of coffee solids in the extract.
Neither solution is particular desirable at producing a stable liquid coffee product. In the first case, when using sodium bicarbonate, the pH still varies upon storage resulting in an inconsistent product because it starts at an undesirably high level, which may still render an unacceptable high pH to the product if it is consumed early in its shelf-life. In the second case, with the excessive alkali and additional neutralization steps, the process tends to over treat the coffee (both due to excessive alkali and neutralization steps), require additional processing steps, and result in additional salts being formed in the beverage (end product of the neutralization) that can negatively effect taste and flavor profiles.